Though present in natural settings at minute quantities, ethylene oxide was first synthesized in a laboratory setting in 1859 by French chemist Charles-Adolphe Wurtz using the so-called “chlorohydrin” process. However, the usefulness of ethylene oxide as an industrial chemical was not fully understood in Wurtz's time; and so industrial production of ethylene oxide using the chlorohydrin process did not begin until the eve of the First World War due at least in part to the rapid increase in demand for ethylene glycol (of which ethylene oxide is an intermediate) as an antifreeze for use in the rapidly growing automobile market. Even then, the chlorohydrin process produced ethylene oxide in relatively small quantities and was highly uneconomical.
The chlorohydrin process was eventually supplanted by another process, the direct catalytic oxidation of ethylene with oxygen, the result of a second breakthrough in ethylene oxide synthesis, discovered in 1931 by another French chemist Theodore Lefort. Lefort used a solid silver catalyst with a gas phase feed that included ethylene and utilized air as a source of oxygen.
In the eighty years since the development of the direct oxidation method, the production of ethylene oxide has increased so significantly that today it is one of the largest volume products of the chemicals industry, accounting, by some estimates, for as much as half of the total value of organic chemicals produced by heterogeneous oxidation. Worldwide production in the year 2000 was about 15 billion tons. (About two thirds of the ethylene oxide produced is further processed into ethylene glycol, while about ten percent of manufactured ethylene oxide is used directly in applications such as vapor sterilization.)
The growth in the production of ethylene oxide has been accompanied by continued intensive research on ethylene oxide catalysis and processing, which remains a subject of fascination for researchers in both industry and academia. Silver catalysts remain today the primary catalytic material for ethylene oxide production, but a number of advances have been made, most notably the efficacy of silver-based catalysts have been made by the addition of small amounts of “promoting” elements such as rhenium and cesium. Nonetheless, despite the extensive research there is still uncertainty over aspects of ethylene oxide catalysis, most notably the role of the silver catalyst and the precise reaction mechanism.
Chlorine has long been used in the feed mixture for the gas phase production of ethylene oxide (see e.g., Law et al., U.S. Pat. No. 2,279,469, issued Apr. 14, 1942; U.K. Pat. No. 1,055,147 issued Jan. 18, 1967, and Lauritzen, EPO Pat. No. 0 352 850 B1, issued Jan. 19, 1994) and has been variously known as an “inhibitor”, “moderator”, “anti-catalyst”, and “promoter”.
While chlorine's role was not fully understood in these prior publications, recent research indicates that chlorine regulates the reaction by withdrawing valence charge from surface-adsorbed oxygen atoms; chlorine is particularly suitable for this because chlorine's affinity for valence electrons is comparable to that of monoatomic oxygen. (See, Richard M. Lambert, Rachael L. Cropley, Alifiya Husain and Mintcho S. Tikhov, Chem. Comm., 2003, 1184-1185). Lower valence charge density of monoatomic adsorbed oxygen makes it a better electrophile, and thus energetically favors “electrophilic attack” on adsorbed ethylene and thus the partial oxidation of ethylene to ethylene oxide. Thus, chlorine plays a key role in maintaining the catalyst's selectivity—the efficiency of the partial oxidation of ethylene to ethylene oxide.
While prior publications have disclosed the use of chlorine under specific conditions, given the importance of chlorine on determining selectivity, and the results of recent studies into chlorine's role in epoxidation, the use of chlorine and the full range of chlorine-containing molecules have not been sufficiently explored. There is thus a continuing need in the art for a suitable chlorine composition for use in olefin epoxidation.